A spinning bicycle is a type of exercise bicycle. A difference of a spinning bicycle from a common exercise bicycle is that, a spinning bicycle is designed to simulate an on-road bicycle, and not only brings better workout effects but also effectively boosts cardiopulmonary performance. Thus, a spinning bicycle is considered an ideal alternative solution for modern people who cannot conduct actual on-road rides. Further, a user of a spinning bicycle is allowed to carry out simulation trainings and thus obtain better results when the user actually rides on-road. In a spinning bicycle, a programmable and controllable continuous braking system is installed between a flywheel and an axis to simulate the feel of actually riding on-road. A conventional braking resistance system is a touch resistance brake, and resistance may be gradually lost due to abrasion with a braking contact plane over an extended period of use. Further, the above conventional braking resistance system provides unstable resistance and requires periodical maintenance and replacement that lead to high maintenance fees. Further, there are devices that employ an electromagnet as a resistance source. Such device, although featuring an advantage of readily adjustable resistance, is quite power consuming. Further, a resistance device employing conventional magnets lacks adjustment flexibilities although being free from the issue of power consumption.
Therefore, there is a resistance source of a resistance device in a brake that uses a magneto-rheological fluid for assisting braking. The above resistance source offers advantages of having stable resistance, no wearable consumables, readily adjustable resistance and low power consumption. The magneto-rheological fluid is a composite fluid, and includes minute magnetic particles, which has high magnetic permeability and a low hysteresis property, and a non-magnetic permeable liquid. The magneto-rheological fluid is filled in a binding gap of a rotor and a stator. When a magnetic line passes the magneto-rheological fluid, the magnetic particles are caused to be arranged and bound in a predetermined direction, in a way that the viscosity of the magneto-rheological fluid is significantly increased to brake the relative rotation between the rotor and the stator. The magneto-rheological fluid is extremely high in performance and produces almost no abrasion and replacement issues. For example, the U.S. Pat. No. 8,397,885 B2, “Magneto-Rheological Fluid Brake”, discloses a magneto-rheological brake resistance device. However, the above disclosure designed with an inner rotor has unsatisfactory torque utilization efficiency and includes multi-polar magnetic coils. As a result, the above disclosure has a complicated structure, gaps present between the multi-polar magnetic coils, poor distribution of magnetic lines and inadequate magnetic permeability efficiency, hence failing to meet application requirements.